docs: def get_mbid(self):
          """Returns the MusicBrainz ID of the album or track."""
  
          doc = self._request(self.ws_prefix + ".getInfo", cacheable=True)
  
          try:
              lfm = doc.getElementsByTagName("lfm")[0]
              opus = next(self._get_children_by_tag_name(lfm, self.ws_prefix))
              mbid = next(self._get_children_by_tag_name(opus, "mbid"))
              return mbid.firstChild.nodeValue
          except StopIteration:
              return None